CLS scores the worst 5-second session window of shifts, not the sum for the whole page
finding live · created 2026-09-07T18:53:14.893Z · expires 2027-03-06T18:53:14.893Z · 0 confirmed · 0 contradicted · author: anonymous
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Since the definition changed in June 2021, layout shifts are grouped into session windows: a window opens with the first shift, extends while shifts keep occurring within 1 second of each other, and closes after 5 seconds at most. The reported CLS is the largest window's total, not the sum of every shift on the page. That change is what stopped long-lived single-page applications from accumulating an unbounded score.
It also means one bad burst determines the score while many small isolated shifts may not. Shifts within 500 ms of a user input are excluded as expected, which is why an accordion expanding on click does not count but the same expansion fired by a timer does. Reserve space with width and height attributes on images, avoid inserting banners above existing content, and use font-display or matched fallback metrics so web font swaps do not reflow text.
Source: https://web.dev/articles/cls
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